1. Field of the Invention
The present invention relates to an improved entrance telephone system of the type utilizing the public subscriber telephone network for signalling, communication and door opening control purposes.
2. Related Prior Art
Such installations are previously known, e.g. from U.S. Pat. No. 3,947,641, in which patent it is disclosed to discriminate subscriber numbers dialled from a conventional telephone set, for example located adjacent to a locked entrance door, thus facilitating establishment of a specific and reciprocal communication between entrance/tenant, and it is further possible for the "entrance set" to receive and interpret a signal generated by the "reply set" as a specific opening command transmitted over the communication circuit established by means of telephone number discrimination. There are various types of installations utilizing this basic principle, for example installations marketed under the trade mark "Telecode" in Sweden, and installation marketed under the trade mark "Entraguard" in the U.S.A., which serve as proof of functionability and usability for these types of installations. Since the established communication originates from the entrance telephone set, having a number filter (hereinafter referred to as number discriminator) which includes the called telephone number and also initiating the call, the called tenant can, when lifting the hand set (B-reply) (i.e., "off-hook"), be informed of the fact that the call originates from the entrance telephone set, while the visitor, who originated the call, may not be made aware of the reply, which makes it possible for the tenant to secretly decide whether or not speech communication should be established with the visitor. In the above mentioned and previously known systems, this is accomplished in such a way, that the entrance set, after having obtained confirmation of B-reply, transmits an identification signal via the public telephone network. This signal is common for all entrance sets, but becomes specific for each calling entrance set. The parts with which a visitor can communicate, i.e. number selecting device and communication unit for speech communication, and any video communication unit, are disconnected when number selection at the entrance has been completed and accepted, and it is the number discriminator, in practical design arranged as a central processor, to which the aforementioned communication units, as well as the telephone network and the entrance lock, can be regarded as peripherally connected, and which provides the actual number selecting procedure to the central office and monitors the network for B-reply indication within predetermined calling time period (approximately 0.5 minute). When and if B-reply is indicated, transmission of an ID-signal (identification signal) commences to the telephone network (from the replying tenant), and a time logic circuit is also connected, which during a time period of approximately 5 seconds monitors any change on the telephone network. Provided that no such change is indicated, the units facilitating communication for a visitor are connected to the already established circuit via the public telephone network, and the visitor is made aware that a reply has been made, and can verbally communicate with the tenant and request unlocking of the entrance door. An entrance call, which can also be utilized for door opening purposes, has thus been established. However, should a change in the telephone network be indicated during the previously mentioned surveillance period (replaced hand set=inverted B-reply, i.e., "on-hook"), said units are not connected, and the circuit established via the public telephone network is disconnected, without the visitor obtaining any knowledge of the fact that a reply has taken place. The visitor is given the impression, that the tenant has not replied. The tenant has thus a possibility, during the time period when the ID-signal is sounded, to secretly make a desired decision whether or not an entrance call should be answered, by deciding between holding the hand set lifted, or by replacing the hand set on the telephone set.
To obtain the above function, it must be possible for the number discriminator to automatically indicate circuit status with regard to B-reply and replaced hand set, i.e. the public telephone network used must be able to provide and transmit such information. However, this is the case substantially only in the countries already mentioned with regard to existing installations, i.e. Sweden and the U.S.A., whereas a change of polarity takes place in Sweden for B-reply and replaced hand set, while in the U.S.A. other methods may be used for these functions, e.g. variations of voltage. Such changes can be detected by the number discriminator, but in other countries, for example Norway, France and the Federal Republic of Germany, there are no such detectable signals. As a result, the above mentioned concept can only be used in countries where it is already being used, but in other countries, the called party can not secretly decide whether or not an entrance call should be switched through, and such calls are thus forced on the called party, which is a disadvantage both with regard to privacy and security.